This invention relates generally to read measurement of resistive memory cells to obtain a cell-state metric for use in cell-state detection during read operations and/or cell programming during write operations. Read measurement methods and apparatus are provided, together with memory devices and cell-state detection and programming methods which employ the read measurement technique.
In resistive memory, the fundamental storage unit (referred to generally herein as the “cell”) can be programmed to any one of s≧2 different states, or levels, which exhibit different electrical resistance characteristics. The s programmable cell-states can be used to represent different data values, whereby data can be recorded in the cells. When reading recorded data, cell-state is detected from measurements on the cells by exploiting the differing resistance characteristics to differentiate between the s possible cell-states. Some resistive memory devices currently offer only single-bit storage. These devices use so-called “single-level cells” which have s=2 programmable cell-states, providing storage of one bit per cell. To increase storage density, however, there is an increasing move toward multibit operation. Multibit memory uses so-called “multilevel” cells which have s>2 programmable cell-states, providing storage of more than one bit per cell.
The class of memory technologies collectively known as resistive random access memory (RRAM) provides a promising example of resistive memory. RRAM has the potential to serve as the “universal memory” that blurs the distinction between storage and memory and can address the growing gap in performance between storage and the rest of a computing system. This will tremendously speed up computers and enable future exascale computing systems. RRAM encompasses various memory technologies including phase change memory (PCM), conductive bridge RAM, and valence change memory. In these technologies, the resistance characteristics of cells are modified via structural and electronic changes of tiny nanoscale volumes of chalcogenides and metal oxides upon application of electrical signals.
In general, resistive memory cells are programmed to different cell-states by the application of current or voltage signals. Read measurements on cells are usually performed by biasing the cell with a fixed read voltage and measuring the resulting current flowing through the cell. The cell current at a fixed read voltage depends on electrical resistance of the cell and hence on cell-state, whereby the resulting fixed-voltage resistance measurement provides an effective cell-state metric. A sufficiently low read voltage is used for this resistance metric to ensure that application of the read voltage does not disturb the programmed cell-state. When the read measurement is performed during a data read operation, the resulting resistance metric is used to detect which of the s possible cell-states each cell is programmed to. Cell-state detection can be performed by comparing the resistance metric for each cell with predetermined reference levels defining the s programmable cell-states. Read measurement can also be performed to check cell-state during write operations. For multilevel cells in particular, programming is usually achieved by means of an iterative write-and-verify scheme involving applying a series of programming pulses, with read measurement after each pulse, so as to converge on the desired cell-state.
The conventional low-field resistance metric has several drawbacks. In the case of PCM, the most significant one is resistance drift whereby the resistance of programmed cell states tends to drift upwardly with time, causing errors in cell-state detection. To counter this, some alternate cell-state metrics have been proposed for PCM. In International Patent Application publication no. WO2012/029007, a metric dependent on slope of the current/voltage characteristic of cells is derived from measurements at a plurality of predetermined read voltages. Another approach is described in “Non-resistance-based Cell-State Metric for Phase Change Memory”, Sebastian et al., Journal of Applied Physics, vol. 110, pp. 084505, 2011 and our copending European Patent Applications numbers 11157698.9 and 11157709.4, filed 10 Mar. 2011. The read voltage is progressively increased until a predefined current threshold is reached, the time taken to reach this threshold providing a time-based metric for cell-state. In spite of their drift tolerance, these approaches can result in reduced signal margin and/or reduced read bandwidth. RRAM technologies other than PCM, on the other hand, commonly suffer from large variability of the high-resistance state, effectively reducing the signal margin.
U.S. Pat. No. 7,885,101 discloses a cell-state detection system for data read operations on N-state PCM cells. The system performs N-1 read measurements, progressing through a series of predetermined read voltages which are fixed for all cells, and compares the cell-current with a predetermined threshold current at each read voltage. The process stops for a given cell when the cell-current is less than the threshold current, this point being determinative of cell-state, i.e., which of the N possible states the cell is programmed to. This is specifically a cell-state detection system, and is not concerned with production of a cell-state metric per se.
There are still distinct advantages in using electrical resistance as a cell-state metric, especially for memory applications where latency is at a premium. Advantages include the simplicity of the read measurement circuitry and the high read bandwidth. It would be desirable, however, to alleviate the drawbacks associated with the conventional resistance metric as discussed above.